The invention relates to envelopes for infrared detectors, to detectors comprising such envelopes and to mounts for cooling elements for use in such envelopes and detectors.
U.S. Pat. No. 2,951,944 describes an envelope for an infrared detector comprising a mounting area in a dewar for mounting at least one detector element for detecting infrared radiation, and an elongate cooling element insertable in the dewar so as to extend towards said mounting area for cooling the detector element during operation. The dewar is at least partially accommodated in a dewar mount, and mounting means are present for securing the cooling element in the dewar. The cooling element and dewar are used to maintain the detector element at a temperature in the cryogenic range so as to obtain more efficient operation of the detector. The cooling element described is of the type which uses the well known Joule-Thomson effect to bleed or throttle the flow of certain types of gases such as dry air, nitrogen, argon, hydrogen, or helium and produce a cryogenic refrigerating effect by expansion of the gas. The transverse dimensions of such a cooling element are usually such that it is a close-fit in the chamber defined by the inner wall of the dewar.
In the evacuated space between the inner and outer walls of the dewar the detector element or element array is mounted on the end face of said inner wall. The elongate cooling element therefore extends to the vicinity of this end face from the mouth of the dewar.
When in use the free end of such a cooling element may tend to move in such a dewar as a result of vibrations in the detector environment and/or of forces caused by, for example, rapid acceleration. The inner wall of the dewar may consist of glass having a thickness of for example 0.5 mm. Particularly when the cooling element is a close fit in the dewar chamber such movement of the cooling element can force the inner end wall of the dewar to move beyond acceptable limits so resulting firstly in undesirable movement of the detector element in the optical field and in an extreme case undesirable straining and cracking of the glass walls of the dewar. Similar straining and cracking of the glass dewar can also occur when inserting and securing the cooling element in the dewar.
In the arrangement known from U.S. Pat. No. 2,951,944, the cooling element mounting means comprises a solid mass of potting compound such as epoxy resin which fills the space between the dewar-mount and the mouth of the dewar. In order to prevent movement of the cooling element relative to the dewar, the cooling element is embedded rigidly in the envelope arrangement by the epoxy resin penetrating and filling the space between the inner wall of the dewar and a metal jacket of the cooling element, except over a short distance from the inner end face of the dewar. This extra metal jacket is necessary to provide a return path for the coolant fluid through the epoxy resin and out from the detector.
However such a mounting has several problems and disadvantages. It is not effective in reducing straining of the dewar when the cooling element is being inserted and secured during assembly of the detector. Due to a difference in thermal expansion coefficients such a large filling or epoxy resin can strain and crack the glass of the dewar either during assembly or in use. If during assembly the cooling element is incorrectly mounted (for example by being spaced incorrectly from the mounting area of the detector element) the whole of the expensive detector envelope arrangement is wasted, because the cooling element which was so thoroughly embedded therein cannot be removed readily. The epoxy resin filling and the metal jacket around the cooling element both of which extend close to the detector element area constitute a significant thermal path and a significant thermal mass. This reduces the speed with which the detector operating temperature can be achieved in this area and/or can require the provision of a larger supply of the coolant fluid than would otherwise be necessary. Furthermore the inner wall of the dewar which is fastened to the cooling element by the epoxy resin may still move and vibrate relative to the outer wall of the dewar during use of the detector, and this movement can still strain the glass of the dewar and vibrate the detector in the optical field.
The arrangement described in U.S. Pat. No. 2,951,944 has a conventional fitting for the fluid connection to the cooling element. Such a fitting is a long flexible metal pipe, a free end of which is connected, in use, to a remote supply of the fluid used for cooling the detector element or elements. The pipe may have a wall thickness of for example 0.15 mm. and an inner diameter of for example 0.5 mm. The length of the pipe may be for example, several centimeters or even several tens of centimeters. Such a long flexible pipe can be damaged comparatively easily.